Linear accelerators require power in the form of high power pulses of short duration. This form of power can be supplied by either a magnetron or a klystron.
Magnetrons are relatively high efficiency, self-oscillating, diode-type electron tubes that are used to produce microwave energy. These electron tubes, which are typically small, light weight and relatively inexpensive, with some models being readily available for purchase, offer peak power levels of up to 5 megawatts (MW) and average power levels of about 10 kilowatts (kW). Power levels, however, are not as high as those offered by klystrons. In addition, magnetrons have a relatively short lifespan (i.e., 3,000 operating hours), cannot easily be rebuilt, and their self-oscillation operation is affected by feedback, especially from highly reactive loads.
Klystrons are specialized vacuum tubes called linear-beam tubes. These tubes offer relatively high power (i.e., up to 30 MW peak power and up to 100 kW average power for an S-band tube, with even higher powers for L-band (1 gigahertz (GHz)) and lower frequency tubes). Tube operation is relatively quiet electrically, but efficiency is low. While these tubes can be rebuilt and offer a relatively long lifespan of up to 20,000 operating hours, they are large and heavy and require a large solenoid. In addition, these tubes are relatively expensive and are not readily available, in some cases requiring delivery times of greater than one or two months.
A need exists for a microwave system to drive a linear accelerator that addresses at least some of the drawbacks associated with these conventional power sources.
The present invention satisfies this need by providing a microwave system for driving a linear accelerator that employs a plurality of magnetrons. More specifically, the inventive system, which offers, among other things, increased magnetron life and improved system reliability, comprises: a plurality of magnetrons; at least one pulse generator to energize the magnetrons; means for synchronizing the frequency and phase of outputs from the magnetrons; and at least one waveguide for transmitting the synchronized outputs or power from the magnetrons to the linear accelerator.
In one embodiment, outputs from the magnetrons are synchronized by arranging at least one pair of magnetrons in parallel and for each such magnetron pair, reflecting a small amount of power from each magnetron in the pair back into the other magnetron, thereby locking their respective outputs. In this embodiment, the inventive microwave system comprises:                (a) at least one pair of magnetrons arranged in parallel and having equal waveguide lengths;        (b) one or two high power pulse generators for each magnetron pair, wherein the pulse generator(s) is connected to one or both magnetrons in the magnetron pair;        (c) coupling means for coupling outputs from each magnetron in a magnetron pair, and optionally for further coupling already coupled outputs from the magnetron pairs; and        (d) at least one waveguide for transmitting coupled output from the coupling means to the linear accelerator, wherein the waveguide has a section with a mismatch for reflecting a small amount of power from each magnetron in a magnetron pair into output from the other magnetron.        
In another embodiment, the magnetrons are synchronized by designating at least one magnetron as a master and one or more remaining magnetrons as slaves and by injecting small amounts of power from the master magnetron(s) into output from the slave magnetron(s). In this embodiment, the inventive microwave system comprises:                (a) a plurality of magnetrons including at least one master magnetron and at least one slave magnetron;        (b) at least one high power pulse generator, wherein the pulse generator(s) is connected to one or more magnetrons;        (c) a coaxial line or waveguide in communication with the master magnetron(s) and the slave magnetron(s) for injecting small amounts of power from the master magnetron(s) into output from the slave magnetron(s); and        (d) at least one waveguide for transmitting the outputs from the magnetrons to the linear accelerator.        
The present invention also provides a radiation (i.e., electron, x-ray) source comprising a linear accelerator, and connected thereto, a microwave system, as described herein above. The inventive radiation source offers increased efficiency and dependability, higher energy and power outputs, as well as, different energy outputs that can take the form of successive pulses that can alternate between at least two different energy levels.
The present invention further provides a method of driving a linear accelerator, the method comprising: employing a plurality of magnetrons; synchronizing outputs from the magnetrons; and delivering the synchronized outputs or power to a linear accelerator.
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and accompanying drawings.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.